Alkylphosphonothioic dihalides have been prepared in the prior art by reacting alkyl halides with phosphorus trihalides in the presence of aluminum chloride, followed by sulfurization of the reaction product. The alkyl halidephosphorus trihalide reaction proceeds at room temperature according to the formula set forth in Heuben-Weyl, Methoden der Organis Chenchemie at vol. 12, part 1 (1965) at p. 396:
Xr--cl + pcl.sub.3 + ALCl.sub.3 .fwdarw. XR--PCl.sub.4 AlCL.sub.3 (I)
The Heuben-Weyl reference also notes that the reaction has been attempted in the absence of the aluminum chloride catalyst with little success. The reaction has the disadvantage that one mole of aluminum chloride is lost for each mole of product prepared.
The reaction product must be sulfurized to obtain the phosphonothioic dihalide product. A. M. Kinnear and E. A. Perren, Journal Chem. Soc., 3437 (1952) showed that (EtPCl.sub.3) (AlCl.sub.4) can be sulfurized with H.sub.2 S. It gave a yield of only 32% EtP(S)CL.sub.2 while obtaining a 47% yield of a by-product EtPS.sub.2.
Alkyl or arylphosphonothioic dihalides also can be prepared according to our U.S. Pat. No. 3,790,629 by reacting an aliphatic or aromatic hydrocarbon with a pentavalent thiophosphorous compound having at least two halogens attached thereto under at least autogenous pressure at a temperature of from 200.degree.C. to 450.degree.C. Also see "Journal of the American Chemical Society", 88:13, July 5, 1966 ". . . A New Synthesis of Phosphonothioic Dihalides," Baker et al, p. 3041.
Cycloalkanephosphonothioic dichlorides have also been prepared by reacting a cycloalkane with thiophosphoryl chloride under irradiation with mercury lamps. Reaction times are long and low yields are reported (Angrew. Chem. Internat. Edit., vol. 9 (1970), No. 6 at p. 458).